Which of these numbers is composite? ${3,\ 11,\ 31,\ 73,\ 99}$
A composite number is a number that has more than two factors (including 1 and itself). 3, 11, 31, and 73 each have only two factors. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 99 is the composite number.